fix backtrace printed when cannot get lock.
change lock error handling code so exceptions have useful info and
exception handling in one place.
add test case for when cannot get lock.
#!/bin/sh
hg init
touch a
hg add a
hg commit -m "Added a" -d "1000000 0"
touch main
hg add main
hg commit -m "Added main" -d "1000000 0"
hg checkout 0
echo Main should be gone
ls
touch side1
hg add side1
hg commit -m "Added side1" -d "1000000 0"
touch side2
hg add side2
hg commit -m "Added side2" -d "1000000 0"
hg log
echo Should have two heads, side2 and main
hg heads
echo Should show "a side1 side2"
ls
hg update --debug -C 1
echo Should only show "a main"
ls